Acrylamide gel electrophoresis of fully reduced and dissociated C3 and C5 demonstrated one alpha and one beta subunit of different molecular weights (140,000 and 80,000 respectively) in each protein. Studies of C3 and C5 that had undergone cleavage by C42-C423 or trypsin demonstrated that C3a and C3b are generated upon cleavage of the C3 alpha subunit. Complement and trypsin exclusively attack the alpha subunit, the cleavage of which underlies the formation of a variety of C5 degradation products, several C5a-like fragments, C5bI, II, III, C5c and C5d. The subunits can now be isolated on a preparative scale after reduction (10 to the minus 2nd power M dithiothreitol) and dissociation. Six molar Guanidine HCl or 0.1% sodium dodecyl sulfate are used as dissociating agents for gel filtration or preparative polyacrylamide gel electrophoresis respectively. Monospecific antisera have been raised against isolated subunits. These antisera are presently employed for the immunochemical characterization of the various C3 and C5 cleavage products. BIBLIOGRAPHIC REFERENCES: Nilsson, U.R., Mandle, R. and Mapes, J.: Human C3 and C5: subunit structure and modifications by trypsin and C42-C423. J. Immunol. 114:815, 1975. Nilsson, U.R., Tsai, C.C., MacArthur, W.P. and Taichman, N.S.: Activation of the complement system by actinomyces viscosus (AV). (Abstract) A.A.D.R., 1975.